While there is a substantial literature highlighting the presence of social dynamics in legislatures, we know very little about the precise processes that generate these social dynamics. Yet, whether social dynamics are due to peer pressure, frequency of interaction, or genuine learning, for example, has important implications for questions of political representation and accountability. We demonstrate how a recent innovation can be used to study the diffusion of behavior within legislatures. In particular, we study diffusion within the US House of Representatives by looking at the dynamic process underlying discharge petitions. The discharge procedure shares many characteristics with other forms of legislative behavior, yet it has one important advantage when it comes to studying social dynamics: we can observe when members decide to sign petitions. Based on data from 1995 to 2014, we find that the social dynamics underlying the discharge procedure tend to involve the rational evaluation of information conveyed by the behavior of previous petition signatories.